This invention relates to modifying the choke of a shotgun barrel and, more particularly, to novel interchangeable choke tubes which can selectively be screwed into the end of a shotgun barrel for providing a range of different choke constrictions for the barrel and a method for preparing the internal surface of a barrel for accommodating such choke tubes.
Shotgun barrels are normally formed with a smooth bore that is either cylindrical along its entire length or has a slight choke constriction at its outer end. Choke modifying devices are available which vary the constriction in the outer end of the barrel for changing the pattern of pellets leaving the barrel. Since each use of a shotgun, from skeet shooting to hunting various types of game, has an optimum choke which controls pellet density and pattern size at a given distance, a shotgun with variable chokes is more versatile than a shotgun with only a single choke.
Many types of choke modifiers have been developed in the past, but they have proven to be less than satisfactory because of their limited applicability, poor performance or impractical expense. For example, many choke modifiers are formed with bulbous appendages which are connected to the outer surface of a barrel. These devices are clumsy and usable only on single-barreled guns and not on double-barrel shotguns of either a side-by-side or over-under design. Another type of choke modifier requires expanding a shotgun barrel to accommodate an insertable tube, which also precludes use in most double-barrel shotguns where the barrels are positioned close together. Other choke modifiers which fit into a counter bore formed in the barrel have been tried, but like many of the other devices mentioned above, satisfactory alignment between the choke modifier and the bore has proven difficult to achieve. Such non-alignment can adversely affect the density of the pellets and their flight pattern and, in addition, cause excessive wear to the barrel and choke modifier.
An attempt to solve this latter problem is taught in U.S. Pat. No. 4,058,925 where the shortcomings of a choke modifier formed independently of a shotgun are discussed. The patent indicates that a number of manufacturing tolerances can affect the alignment of the choke with the bore, such as (1) angularity of threads in a counter bore to the barrel bore; (2) angularity of the outside diameter of a choke tube to the inside diameter of the tube; (3) tolerances of thread pitch diameter on both barrels; (4) concentricity of bore to counter bore in barrel; (5) concentricity of outside diameter of choke tube to inside diameter of choke tube; (6) diameter of bore; and (7) diameter of choke in tube. Inaccuracies in these tolerances are indicated as causing the bore of the choke tube not to be aligned with the bore in the shotgun barrel.
U.S. Pat. No. 4,058,925 proposes to eliminate these tolerance problems by forming a permanent choke constriction in a barrel and during the same operation form a series of detachably mounted, progressively reduced choke tubes which can selectively be removed to form different choke constrictions. By forming the barrel and choke tubes together the tolerance inaccuracies are supposedly reduced to the point where alignment is satisfactory. The obvious disadvantage of this procedure, however, is that choke tubes can only be used with one shotgun, the one they were formed with, and the method cannot be used to form choke modifiers for the many shotguns already in use. Further, if one or more of the tubes become lost or damaged, usable replacements cannot be made. It is also believed that satisfactory alignment is not totally achievable by this method because if the choke tubes are formed before the barrel is completed (including adding the bluing to the barrel) slight distortions occur during final stages of manufacturing which can cause alignment problems.